1. Field of the Invention
The present invention relates generally to stabilized liquid films and, more particularly, to stabilized liquid membranes for separations in the gaseous phase.
2. Description of the Prior Art
Membrane systems for the separation of gases are potentially attractive because they offer low capital and operating costs, along with low energy requirements. Unfortunately, the high performance selectivity required for most applications cannot be met by prior art membranes. The major obstacle to be overcome is the development of stable membranes which simultaneously have the high selectivities and fluxes required for energy efficient operation. A performance summary comparison of several prior art membrane types with membranes of the present invention appear in Table I, next below.
__________________________________________________________________________ PERFORMANCE SUMMARY OF VARIOUS SELECTIVE MEMBRANES FOR GAS SEPARATIONS AREA IN WHICH PERFORMANCE CRITERIA ARE SATISFACTORY FOR MOST APPLICATIONS INTERNAL RESISTANCE MEMBRANE TYPE SELECTIVITY PERMEABILITY THICKNESS STABILITY __________________________________________________________________________ CONVENTIONAL POLYMER X MEMBRANE (PRIOR ART) ANISOTROPIC POLYMER X X MEMBRANE (PRIOR ART) CONVENTIONAL IMMOBILIZED X X LIQUID MEMBRANES (PRIOR ART) STABILIZED LIQUID X X X MEMBRANES ANISTROPIC STABILIZED X X X X LIQUID MEMBRANES __________________________________________________________________________
The first three membrane types in the Table are found in the prior art. Generally, those prior art membranes which are stable lack selectivity and are not sufficiently permeable and those exhibiting selectivity and desirable permeability are not stable.
Conventional solid polymer membranes, for example, have been widely investigated for gas separations for many years. In order to maximize transport fluxes, asymmetric versions of polymer membranes with very thin (approximately one micron) membrane "skin" have been developed. Examples of these are found in Riley, et al, J. Appl. Polymer Science, 1967, 11, 2143 and S. Lobe, et al, Saline Water Conversion II, p. 117, Advances In Chemistry Series No. 38, American Chemical Society, 1962. While representing progress in the field, these ultra-thin polymer membranes are still short of the flux needed for many applications. Also, the selectivity of the membranes to gases has often been poor and this limits their usefulness.
Another approach to achieving high fluxes, beside making the membranes thinner, is to use materials which have high permeability. Liquids are much more permeable than solid polymers. This is because of the high gas diffusion coefficients in liquids (approximately 1,000 times greater than in solid polymers) and the high gas/vapor solubilities in liquids. Immobilized liquid membranes (ILMs), which are composed of liquids immobilized in porous polymer matrices such as those described by W. I. Ward, et al in Science, 1967, 156, 1481, not only have high permeabilities, but are also simultaneously highly selective unlike polymer membranes.
Although immobilized liquid membranes are much more permeable and selective than solid polymer membranes and have shown a great deal of promise, such prior art membranes have several deficiencies which have to be overcome before they can be effectively utilized. The most important is that they are not stable over long periods of time. The short life time of such membranes is primarily due to the loss of the liquid phase by evaporation. As a practical matter, then, such prior art liquid membranes can be used only if the gas stream flowing past the membrane is first treated, preferably saturated, with the liquid used in the membrane to reduce evaporative loss, or, if the membrane liquid is replaced frequently.
However, as the above illustrates, a need has existed for an ILM system of reduced vapor pressure which, at the same time, maximizes flux.